1. Technical Field
This disclosure relates to a droplet ejection device and an image forming apparatus including the droplet ejection device, and more specifically to a maintenance structure for a droplet ejection unit.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multifunction devices having two or more of the foregoing capabilities. As one type of image forming apparatuses, for example, inkjet recording apparatuses employing liquid ejection recording methods are known that use a recording head(s) for ejecting droplets of liquid (e.g., ink).
The inkjet recording apparatuses employing liquid ejection recording methods eject ink droplets from a recording head(s) onto a recording medium (also referred to as recording sheet), e.g., a sheet of paper, to form (record or print) images on the recording medium. Such inkjet recording apparatuses fall into two major types: a serial type in which, while the recording head moves in a main scanning direction, the recording head ejects liquid droplets to form images, and a line type in which a line-type stationary recording head ejects liquid droplets to form images.
The liquid ejection methods include, for example, the following methods
In one method, for example, a piezoelectric actuator vibrates and deforms a portion of a wall of a liquid chamber filled with ink to increase pressure in the liquid chamber to eject ink. In another method, for example, a heating element to generate heat upon energization is provided within a liquid chamber, and bubbles generated by heat of the heating element increase pressure of the liquid chamber to eject ink.
Such inkjet recording apparatuses are widely used because of advantages, for example, high speed and less noise, less constraints on the types of recording media including recording sheets of paper, and easiness of color printing.
Here, the above-described serial type and line type are further described below. The serial type of inkjet recording apparatuses typically has a carriage mounting a droplet ejection head. The carriage is serially moved for scanning in a direction perpendicular to a transport direction of a recording sheet, and the recording sheet is intermittently transported in accordance with a recording width. Thus, transport and recording (i.e., droplet ejection) can be alternately repeated.
The line type of inkjet recording apparatuses can employ a line head having droplet ejection nozzles arrayed corresponding to a whole area of one edge of the recording sheet. Unlike the serial type, the line type transports a recording sheet without moving a droplet ejection head, which is more advantageous in enhancement of the recording speed than the serial type.
Such an inkjet recording apparatus may perform maintenance and recovery operation on a recording head unit used as the above-described droplet ejection head to stabilize ink ejection from the recording head unit. In other words, by maintaining a state in which a liquid ejection face (also referred to as nozzle face) of the recording head unit is free from dried, solidified, or viscosity-increased residual ink, such an inkjet recording apparatus prevents residual ink on the nozzle face from hampering ink ejection from the recording head unit and also prevents bubbles from causing ejection failure, such as a reduced position accuracy of droplets landed on a recording medium.
Hence, for example, a nozzle-performance maintenance assembly is proposed to maintain and recover a normal state of an ink ejection performance of such a droplet ejection head. The nozzle-performance maintenance assembly has, for example, a capping function to cover the nozzle face with a moisture retention cap of a high sealing performance to prevent viscosity increase or firm adherence of ink by minimizing vaporization of ink, a discharge recovery function to discharge ejection failure factors, e.g., bubbles in nozzle orifices, by refilling and pressure feeding of recording liquid, a wiping function to wipe residual ink on the nozzle face, which may affect a flying state of liquid droplets, and a dummy ejection function to eject ink droplets to prevent drying of nozzles that are not used for image formation.
To suck ink from ejection nozzles and discharge bubbles in such a maintenance assembly, for example, a cap may have an ink collection port and an air release port connected to a suction channel and an air release channel (see, e.g., JP-2000-211164-A or JP-2007-190845-A). For example, JP-2000-211164-A describes a configuration in which a cap has an ink collection port, a suction port connected to the ink collection port and communicated with a waste liquid tank, a suction pump mounted on the suction channel, an air release channel connected to the air release port and communicated with an exterior of the cap, and an air release valve near the cap in the air release channel.
For such a configuration, maintenance and recovery operation is performed on ejection nozzles according to, for example, the following procedure.
First, the cap is brought into close contact with ejection nozzles and the air release valve is closed to seal the inside of the cap. In such a state, the suction pump is activated to suck ink or bubbles from the ejection nozzles, and the air release valve is opened to release negative pressure in the cap. In such a state, the suction pump is activated to feed ink accumulated in the cap toward the waste liquid tank.
In such a configuration as described in JP-2000-211164-A in which, by opening and closing the air release channel with the air release valve the inside of the cap is turned into negative pressure and returned to atmospheric pressure, pressure inside the cap is smoothly changed toward atmospheric pressure when the air release valve is opened. However, the inventor has recognized that, when the cap is detached from ejection nozzles, in other words, decap operation is performed with the air release valve open or ink suction is performed, ink may move into the air release channel or toward the air release valve.
In other words, when the cap having a space of negative pressure is detached from the ejection nozzles, a rapid change in pressure may wave a surface of ink in the cap, thus causing a portion of ink to move beyond the air release port into the air release channel.
If ink enters and firmly adheres in the air release channel, the channel area may be changed. In such a case, when the air release valve is opened, the flow of air may be hindered, thus hampering smooth return of the inside of the cap to atmospheric pressure when the cap is decapped from the ejection nozzles. As a result, the efficiency of collection of ink into the waste liquid tank by suction of ink may be reduced or liquid leakage may cause contamination of a surrounding area. In addition, adherence of ink on the air release valve may hamper normal operation of the air release valve. As a result, if opening and closing timings or the release amount of air is shifted, the above-described failures may arise.
Alternatively, when ejection nozzles are arrayed in a horizontal direction instead of the above-described vertical direction, the surface of ink is likely to become higher than the air release port. Hence, to prevent ink from entering the air release channel in such a configuration, for example, JP-2000-211164-A proposes a configuration in which the air release port is disposed at a position higher than a suction port in the vertical direction so that the surface of ink does not touch the air release port.
For such a configuration, however, the inventor has recognized that, as described above, if the ink surface is waved or bubbled in decap operation, the ink surface may not be stabilized, thus hampering reliable prevention of entry of ink into the air release port.